Method of cementing casings in oil wells



18, 1953 w. K. WILLIAMS ET AL 2,649,160

METHOD OF CEMENTING CASINGS IN OIL WELLS Filed March 15, 1952 VMM mwm mm w mm m f m m Patented Aug. 18, 1953 UNITED stares e TENT ()FFlCE Dallas, Tex., assignors to The AtlanticRefining Company, Philadelphia, ha a corporation of Pennsylvania Application March 15, 1952, Serial N o. 276,812 8 Claims. (01.166-22) This invention relates to improvementsin cementing well conduits in bore holes and more particularly is concerned with a process for forming. a substantially fluid impervious bond between the wall of the bore hole and a well casing or the like inserted in the bore hole whereby to eliminate the necessity of removing the filter cake, formed during drilling of the bore hole, from the wall of the bore hole.

In completing oil and gas wells, or the like, it is customary, after drilling has been completed, to run a casing into the well at least to the depth of the producing formation. The casing is perforated at the level of the producing formation in order to permit the flow of oil into the casing from said formation. It is necessary to seal off the portion of the bore hole traversing the producing formation from the remainder of the bore hole in order to prevent the flow of undesirable extraneous fiuids into the casing through its perforations. This is normally accomplished by introducing into the well casing a cement slurry which is pumped downwardly therethrough to the bottom of the well where it passes into the annular space between'the exterior of the casing and the producing formation. If desired, of course, suilicient cement slurry may be pumped through the casing to fill the annular space between the casing and bore hole for the entirelength of the casing.

After the cement has hardened the casing and surrounding cement sheath are perforated at the level of the producing zone, this perforating being accomplished through the use of any one of a number of well known perforating devices.

The cement sheath in the annular space between the casing and wall of 'the'bore' hole is intendedto prevent the fluids in formations above and below the producing zone from flowing vertically within the annular space into thearea of the producing zone. In actual practice,however, the heretofore known methods of bonding cming to the wall of a bore hole have resulted merely in retarding such vertical flow, and. in some cases are almost completely inefiective. This is due in many cases to the presence of the layer of mud, commonly called a filter cake, which is deposited on the wall of a borehole during the drilling thereof by the well known rotary drilling method involving the use of a mud laden drilling fluid. As is well known to those skilled in the art, such filter cakeperforms very important func-- tions during the drilling operations; however, during subsequent cementing operations the-filter cake prevents the cement from coming into .di- 3 rect contact with thewalls of the bore hole and forming ,a, bond therewith. Furthermore, the filter cake itself is not mechanically stable, and is subject to penetration and erosion by water within the bore hole. Thus, although the cement substantially fills the annular space between the casing and the borehole wall there is still frequently undesirable communication between the producing formation and other formations traversed by the bore hole due to leakage through the filter cake and between the filter cake and the wall of the bore hole. 7

Heretofore, the most common methods of dealing with the above problem have been to attempt to mechanically scrape away the filter cake from that portion of the wall to which it is desired to bond the casing, or to wash away the filter cake by subjecting it to the erosive action of high pressure jetsof water, or to employ a combination of mechanical scraping and washing. These methods, however, have met with only partial success due to the fact that it is almost impossible to remove all traces of the mud cake, and the presence of even a minute film is suificient to prevent a good bond between the cement and the formation. I

Accordingly, it is one object of this invention to provide a method of bonding a well conduit. to the wall of a bore hole without the necessity of first removing the filter cake from the wall of the bore hole. v

Another object of the invention is to provide a method of forming a fluid impervious bond between a 'well conduit and a bore hole having a filter cake deposited on the wall thereof, in which the filter cake is converted into a cementitious mass, and then cement is introduced between said mass and thewell conduit.

Other objects and featuresof the invention will be apparent from the description which follows and the appended drawing.

In the drawing, Figure 1 is a sectional view of a bore hole having 'a casing bonded therein in accordance with the present invention.

Figure 2 is a detailed section of a portion of the bore hole and casing shown in Figure 1.

It has been found thata fluid impervious bond may be formed between the wall of a bore hole and a well casing positioned in the bore hole by converting the filter cake, deposited on the bore hole wall during conventional rotary drilling operations, into a cementitious mass and then forming a bond between said mass and the well casing byintroducing therebetween cement such as is commonly used in'completion of oil andgas wells. In accordance with this invention, the desired bore hole is drilled using a conventional drilling fiuid to which has been added a first chemical reagent whereby there is deposited on the bore hole wall a filter cake containing such first reagent. A liquid solution containing a second chemical reagent is introduced into the bore hole in contact with the filter cake and sub jected to pressure to cause a portion of such liquid to penetrate the filter cake. The first and second reagents are chemical compounds capable of reacting to convert the filter cake into a hard cementitious mass or materialwhich is intimately bonded to the formation and which is capable of being bonded in turn to cement of the type conventionally used in well completion, the cement being introduced into the annular space between the filter cake and the casing either concurrently with the liquid solution, or subsequent thereto.

By the process of the present invention, the filter cake formed on the bore wall during drilling is converted into a cementitious mass or material which, in addition to functioning as an intermediate material for bonding the conventional cement between the bore hole wall and the well casing, is itself a stable material capable of resisting channeling and erosion. Thus, by this novel process the problem of channeling of well liquids between the bore hole wall and the well casing, frequently present in the case of cement bonds formed by heretofore known processes, is obviated.

The first reagent which is incorporated into the drilling fluid and the second reagent which is subsequently brought into contact with the filter cake to convert the latter into the cementitious mass or material may be any pair of chemical compounds capable .of reacting to form an oxy-acid cement which cements are well known in the art as Sorel cements (Colloid Chemistry, volume II, Alexander, 1931, p. 365). There are many such reagents which may be reacted to form Sorel cements. By way of example, one of the reagents may be an oxide or. hydroxide of a metal such as magnesium, calcium, zinc, barium, beryllium, strontium, cadmium, ferric iron, cobalt, nickel, and cupric copper, or mixtures thereof. The other reagent may be a strong mineral acid salt of the same metals such as magnesium Y chloride, magnesium sulfate, magnesium nitrate, calcium chloride, calcium sulfate, calcium nitrate, zinc chloride, zinc sulfate, zinc nitrate, barium chloride, barium sulfate, barium nitrate, beryllium chloride, beryllium sulfate, beryllium nitrate, strontium chloride, strontium sulfate, strontium nitrate, cadmium chloride, cadmium sulfate, cadmium nitrate, barium chloride, barium sulfate, barium nitrate, ferric chloride, ferric sulfate, ferric nitrate, cupric chloride, cupric sulfate, and cupric nitrate, or mixtures thereof. The two reagents may be compounds of the same metal or metals or of different metals, best results being obtained, however, when using compounds of the same metal or metals.

The drilling fluid used in the process of the present invention is formed by adding a quantity of the first reagent to a suitable conventional water base drilling fluid. Conventional drilling fluids comprise suspensions of clay, usually bentonite, in Water containing one or more additives such as caustic, quebracho, and starch, dependent upon the particular bore hole conditions. It is necessary only that the conventional drilling fluid be free of any material that would react with the first additive or otherwise prevent the conversion of the filter cake into a cementitious mass by reaction of the first and second reagents.

The concentration of the first reagent in the drilling fluid may vary over a wide range and is dependent, among other things, on the nature of the drilling fluid to which it is to be added, and the particular chemical compounds employed as the reagents. It has been found, however, that the first reagent should comprise at least 10% of the solid components of the drilling fluid in order to produce a good bond, and if desired quantities of the first reagent up to 30% and higher may be employed with good results.

The concentration of the second reagent in the liquid solution, preferably water, is not critical but the strength of the cementitious material into which the filter cake is converted increases with concentration. An aqueous solution containing from 15% to 50% by weight of the second reagent has been found to be most satisfactory.

In the drawing there is shown a casing bonded to the wall of a bore hole in accordance with the invention. Numeral I indicates generally a bore hole which has been drilled into the earth through a plurality of formations including the producing formation generally indicated by the numeral 2. Mounted in bore hole I is a casing 3 which may be suspended therein, as, for instance, by means of casinghead 4 located at the surface of the earth 5. The lower end of easing 3 is bonded with the Wall 6 of that portion of the bore hole I which passes through formation 2, so as to prevent the flow through the bore hole of fluids into the zone of formation 2 from formations above or below this formation. The numeral 1 indicates the cementitious material which is formed from the filter cake in the manner set forth in more detail hereinbelow, and which forms an intimate bond with the wall of bore hole I passing through formation 2.

Numeral 8 designates a Portland-type cement which may be of any desired composition, but which is preferably a conventional oil well cement, of the type provided by the cement industry for use in oil wells. Such cement, as is well known to those skilled in the art, is a modified Portland cement containing an increased amount of iron oxide and other additives whereby to make it more suitable than ordinary Portland cement for use in oil wells.

The cement 8 forms a tight bond with the exterior of easing 3 and with the interior surface of the annular body of cementitious material 1. Figure 2 is provided to show in more detail the bond between the wall 5 of formation 2, the cementitious material 7, the oil well cement 8, and the wall of casing 3. It will be appreciated, of course, that the relative radial thicknesses of the sheath of cementitious material I, the annular body of Portland-type cement 8, and the Wall of easing 3, as shown in Figures 1 and 2 are representative only, and that they may vary considerably from the relative dimensions shown.

By way of example, in accordance with this invention, the bore hole through the formation to which the casing is to be bonded is drilled by the conventional rotary drilling method, such as is well known to those skilled in the art, using the special drilling fluid containing the first reagent. The solid components of this drilling fluid may consist of the following: bentonite 60%; the first reagent, magnesia (either hydrated or unhydrated), 30%; sodium hydroxide 3%; que- -When the well has been drilled to the desired depth, drilling is stopped, the drilling string withdrawn, and the casing introduced into the well in the conventional manner. After the casing has been run in, an aqueous solution of magnesium chloride, for instance, a solution containing by weight of magnesium chloride, is introduced into the casing and passed downwardly therethrough to the bottom of the well and thence upwardly into theannulus between the casing and the wall of the bore hole whereby it will be brought in contact with the filter cake. Sufficient pressure is maintained on the well by any method known to those skilled in the art in order to insure that the pressure exerted by the magnesium chloride solution standing in the well is greater than the formation pressure existing in the formation to which the casing is to be bonded. Under such conditions it will be appreciated that the magnesium chloride solution will penetrate and pass through the filter cake in much the same manner as the liquid components of the drilling fluid passed therethrough during the drilling operation.

The magnesium chloride solution is permitted to stand in the well for a sufficient length of time to insure complete penetration of the filter cake. It has been found that 10 minutes is a sufi'icient period of time to accomplish this purpose. Thereafter, water is circulated down through the casing and up through the annulus to remove the excess magnesium chloride solution.

After the excess of the magnesium chloride solution has been removed, a slurry of the Portland-type cement known as oil well cement is pumped into the casing to the bottom thereof and up into the annulus between the exterior of the casin and the filter cake, and permitted to set.

When the method as set forth above is followed, the magnesium chloride, upon passing through the filter cake, reacts with the magnesia contained in the filter cake, and the filter cake is thus converted into a cementitious material comprising magnesium oxychloride cement, with the other components of the filter cake acting somewhat as aggregate materials. The cementitious material formed in the manner set forth above forms a firm bond with the wall of the bore hole, and furthermore, is capable of forming a tight impervious bond with Portland cement or oil well cement which is introduced subsequently into the bore hole.

Since the cementitious material is intimately bonded to the wall of the bore hole and is also bonded to the conventional oil well cement, which in turn forms a watertight bond with the exterior of the well casing, it is seen that a bond is formed between the exterior of the well casing and the wall of the bore hole which precludes the vertical passage of fluids from formations above or bonded.

and up through the annulus.

The; composition of the drilling fluid used in the above example is intended to be representative only since wide variations in composition are possible. For instance, it has been found that a drilling fluid havin as low as 10% of its solid components comprised of magnesia, forms- "a satisfactory filter cake for the carrying out of this invention. Likewise, the percentage of magnesia may be increased to any desired extent consistent with maintaining the drilling fluid in satisfactory condition as regards water loss, viscosity, thixotropic qualities, etc.

'centration of the magnesium chloride solution is Also, the connot critical, it being found, however, that the strength of the resultant cementitious material increases with concentration. It is also necessary for best results to use an excess of magnesium chloride in order to insure a complete reaction between the filter cake and the magnesium chloride solution. It has been found, however, that a solution containing from 15% to 50% by weight of magnesium chloride dissolved in water is most satisfactory.

While the above example is given to illustrate the forming of a bond in which the filter cake is converted into magnesium oxychloride cement, the oxy-acid cements of metals other than magnesium will perform satisfactorily for this purpose, as hereinbefore pointed out. For instance, the drilling mud used in drilling through the formation to which the casing is to be bonded may contain 10% or more of lime (either hydrated or unhydrated), and a solution of calcium chloride may be brought into contact with the filter cake after the drilling is completed and the casing set to convert the filter cake into calcium oxychloride cement. Likewise, an oxide or hydroxide of zinc, barium, cadmium, iron, copper, or a number of other metals or mixtures thereof may be included in the drilling mud whereby'a filter cake may be formed which may be converted into the corresponding oxy-acid cement by later bringing into contact with the filter cake a solution of a water soluble salt of the same metal or mixtures of metals. Also, complex cementitious materials may be formed, as for instance, by including a mixture of magnesia and ferric oxide in the drilling fluid and passing a solution containing magnesium chloride and ferric chloride through the filter cake thus formed.

Furthermore, it has been found that a filter cake containing a sufficient amount of the oxide or hydroxide of a first metal, for instance magnesia, may be converted into a cementitious material by passing therethrough a wash solution comprising the soluble salt of a second metal, for instance, calcium chloride. As a further example it has been found that a solution of barium chloride will react to convert into cement-like material filter cakes containing magnesia, lime, cupric oxide, or ferric oxide in the required amounts. However, best results are obtained when the oxide (or hydroxide) in the filter cake and the salt in the wash solution are compounds of the same metal.

In another embodiment of the invention, a bore hole is drilled through the formation to which the casing is to be bonded using a drilling fluid, the solid components of which include 10% by weight of lime. After drilling is completed, and the casing run into the bore hole, the excess of the drillin fluid is washed out of the bore hole by circulating water down through the casing Thereafter, a cement slurry of special composition, as explained below, is pumped down through the casing and up into the annulus where it is maintained until set. This cement slurry is made by mixing with the dry cement an aqueous solution of magnesium chloride. The amount of magnesium chloride used in the solution is preferably equal in weight to about one-tenth (-1 6) the weight of the dry cement, and a suilicient amount of the solution is employed to bring the cement slurry to pumpable consistency. Filtrate from the cement slurry prepared as set forth above comprises an aqueous solution of calcium chloride. The exact nature of the reaction between the Portlandtype oil well cement and the magnesium chloride is not known; however, it has been observed that the filtrate from the cement slurry formed by mixing an aqueous solution of magnesium chloride with the dry cement comprises calcium chlo ride.

The filtrate, calcium chloride solution, from the cement slurry penetrates the filter cake containing the lime and converts into calcium oxychloride cement which bonds both to the Portland type cement introduced into the annulus and to the wall of the formation against which the filter cake was formed.

While in the above example magnesium chloride was illustrated as having been added to the cement slurry to cause a calcium chloride filtrate therefrom, it is to be understood that any other material, which when added to a Portland-type cement slurry, will give a calcium chloride filtrate therefrom, may be used. For example, it has been found that ferric chloride, cupric chloride, and the chlorides of many other materials will cause the filtrate from Portland type cement to be in the form of an aqueous solution of calcium chloride. On the other hand, it has been found undesirable to add calcium chloride to the Portland-type cement slurry since the cement will tend to flash set if calcium chloride is added thereto in sufficient to react in the desired manner with the filter cake.

The reason why Portland-type cement does not flash set in the presence of the calcium chloride filtrate produced by adding magnesium chloride (or other chlorides) thereto, while it does flash set in the presence of a calcium chloride solution added directly thereto is not understood. However, this phenomenon has definitely been observed, and it is speculated that the flash set in the latter case is caused by an excess of calcium which, of course, is not increased by the addition of, for example, magnesium chloride.

In a variation of the above described method of bonding a casing in a bore hole, the drilling fluid may contain barium oxide and a solution of barium chloride may be used as the liquid component of the cement slurry. It has been found that the filtrate from such cement slurry will not be in the form of a solution of calcium chloride as when magnesium chloride is employed, but rather remains in the form of barium chloride solution. Thus, according to this methd, the filter cake containing barium oxide is converted into a cementitious material comprising barium oxychloride which is bonded both to the wall of the bore hole and to the Portland-type cement when set.

While in the examples given above the soluticn used for converting the filter cake into a cementitious material has been represented as being the chloride (for instance, magnesium chloride, calcium chloride, etc.),, as. hereinbefore pointed out, other soluble salts of the metals may be substituted for the chloride salt in the aqueous solution with more or less satisfactory results. For instance, when the filter cake is formed by a drilling fiuid containing magnesia, the solution used to convert the filter cake to an oXy-acid cement may be a solution of magnesium sulfate, thus resulting in the formation of magnesium oxysulfate cement. Likewise, magnesium nitrate may be used and the filter cake thereby converted into magnesium oxynitrate cement.

While preferred embodiments of this invention have been described in detail above, it will be appreciated by those skilled in the art thatnumerous minor changes may be made without departing from the spirit and scope of the invention. For instance, under certain conditions it may be desirable to introduce the salt solution into the well through the drilling string whereby to convert the filter cake into cement-like material prior to withdrawing the drilling string and insorting the casing. Likewise, it may be desirable to bond the casing to the wall of the bore hole throughout the entire length thereof, in which case the special drilling fluid would be employed throughout the entire drilling operation. Therefore, it is to be understood that this invention is not limited to the specific examples set forth above, but rather is limited only in accordance with the appended claims.

We claim:

1. The method of forming a bond between the wall ofa formation penetrated by a bore hole and the exterior of a well casing positioned in said bore hole which comprises drilling through said formation using a drilling fluid containing a first reagent whereby there is deposited on the wall of said formationa filter cake containing said first reagent, positioning said casing in said bore hole in a manner to form an annular space between said filter cake and said casing, introducing into said annular space and in contact with said filter cake a solution of a second reagent, subjecting said solution to suflicient pressure to cause a portion thereof to penetrate said filter cake, removing the excess of said solution, and introducing into said annular space cement capable of bonding to the exterior of said casing, said first and second reagents being chemical compounds capable of reacting to convert said filter cake into a cementitious material thereby providing a fluid impervious bond with the wall of said formation and with said cement in said annular space.

2. The method according to claim 1 in which said first and second reagents are chemical com pounds which react to form a Sorel cement.

3. The method according to claim 1 in which said first reagent is magnesia and said second reagent is magnesium chloride.

4. The methodaccording to claim 1 in which said firstreagent is lime and said second reagent is calcium chloride.

5. The method of forming a bond between the wall of a formation penetrated by a bore hole and the exterior of a well casing positioned in said bore hole which comprises drilling through said formation using a drilling fluid containing a first reagent whereby there is deposited on the wall of said formation a filter cake containing said first reagent, positioning said casing in said bore hole in a manner to form an annular space between said filter cake and said casing, introducing into said annular space cement containing in solution a second reagent, and subjecting said cement to suflicient pressure to cause a portion of said second reagent to penetrate said filter cake, said first and second reagents being chemical compounds capable of reacting to convert said filter cake into a cementitious material.

6. The method according to claim 5 in which said first and said second reagents are chemical compounds which react to form a Sorel cement.

7. The method according to claim 5 in which said first reagent is magnesia and said second reagent is calcium chloride.

8. The method according to claim 5 in which said first reagent is lime and said second reagent is calcium chloride.

WILLIAM K. WILLIAMS. MILTON E. KROGH.

References Cited in the file of this patent UNITED STATES PATENTS 10 Number Name Date 2,156,220 Dunn Apr. 25, 1939 2,609,881 Warren Sept. 9, 1952 

1. THE METHOD OF FORMING A BOND BETWEEN THE WALL OF A FORMATION PENETRATED BY A BORE HOLE AND THE EXTERIOR OF A WELL CASING POSITIONED IN SAID BORE HOLE WHICH COMPRISES DRILLING THROUGH SAID FORMATION USING A DRILLING FLUID CINTAINING A FIRST REAGENT WHEREBY THERE IS DEPOSITED ON THE WALL OF SAID FORMATION A FILTER CAKE CONTAINING SAID FIRST REAGENT, POSITIONING SAID CASING IN SAID BORE HOLE IN A MANNER TO FORM AN ANNULAR SPACE BETWEEN SAID FILTER CAKE AND SAID CASING, INTRODUCING INTO SAID ANNULAR SPACE AND IN CONTACT WITH SAID FILTER CAKE A SOLUTION TO SUFFICIENT PRESSURE TO SUBJECTING SAID SOLUTION TO SUFFICIENT PRESSURE TO CAUSE A PORTION THEREOF TO PENETRATE SAID FILTER CAKE, REMOVING THE EXCESS OF SAID SOLUTION, AND INTRODUCING INTO SAID ANNULAR SPACE CEMENT CAPABLE OF BONDING TO THE EXTERIOR OF SAID CASING, SAID FIRST AND SECOND REAGENTS BEING CHEMICAL COMPOUNDS CAPABLE OF REACTING TO CONVERT SAID FILTER CAKE INTO A CEMENTITIOUS MATERIAL THERBY PROVIDING A FLUID IMPERVIOUS BOND WITH THE WALL OF SAID FORMATION AND WITH SAID CEMENT IN SAID ANNULAR SPACE.
 2. THE METHOD ACCORDING TO CLAIM 1 IN WHICH SAID FIRST AND SECOND REAGENTS ARE CHEMICAL COMPOUNDS WHICH REACT TO FORM A SOREL CEMENT. 